The present invention relates to multilayer optical interference films, and more particularly to optical interference films which reflect light in the infrared region of the spectrum while transmitting substantially all light in the visible spectrum.
Coextruded multilayer films have been made which comprise multiple alternating layers of two polymers with individual layer thicknesses of 100 nanometers or less. Such multilayer films are described, for example, in Alfrey et al, U.S. Pat. No. 3,711,176. When polymers are selected to have a sufficient mismatch in refractive indices, these multilayer films cause constructive interference of light. This results in the film transmitting certain wavelengths of light through the film while reflecting other wavelengths. The multilayer films can be fabricated from relatively inexpensive and commercially available polymer resins having the desired refractive index differences. The films have the further advantage in that they may be shaped or formed into other objects.
The reflection and transmission spectra for a particular film are primarily dependent on the optical thickness of the individual layers, where optical thickness is defined as the product of the actual thickness of the layer times its refractive index. Films can be designed to reflect infrared, visible, or ultraviolet wavelengths of light depending on the optical thickness of the layers. When designed to reflect infrared wavelengths of light, such prior art films also exhibit higher order reflections in the visible range, resulting in an iridescent appearance for the films. The films produced in accordance with the above mentioned Alfrey patent exhibit iridescence and changing colors as the angle of incident light on the film is changed.
For some applications, while reflection of infrared wavelengths is desirable, higher order reflections of visible light are not. For example, infrared reflecting films can be laminated to glass in buildings and automobiles to reduce air conditioning loads. The films may also be laminated to other substantially transparent plastic materials to reflect infrared wavelengths. However, the films must be substantially transparent to visible light so that the vision of those looking through the glass or plastic is not impaired.
It is possible to suppress some higher order reflections by proper selection of the optical thickness ratio in two component multilayer films. See, Radford et al, Reflectivity of Iridescent Coextruded Multilayered Plastic Films, Polymer Engineering and Science, vol. 13, No. 3, May 1973. However, it is not possible to suppress two successive higher order reflections with two component films.
Other workers have designed optical coatings comprising layers of three or more materials which are able to suppress certain higher order reflections. For example, Thelen, U.S. Pat. No. 3,247,392, describes an optical coating used as a band pass filter reflecting in the infrared and ultraviolet regions of the spectrum. The coating is taught to suppress second and third order reflectance bands. However, the materials used in the fabrication of the coating are metal oxide and halide dielectric materials which must be deposited in separate processing steps using expensive vacuum deposition techniques. Also, once deposited, the coatings and the substrates to which they are adhered cannot be further shaped or formed. Further, the coatings are subject to chipping, scratching, and/or corrosion and must be protected. Finally, because vacuum deposition techniques must be used, it is difficult to fabricate coatings which cover large surface areas.
Rock, U.S. Pat. No. 3,432,225, teaches a four layer antireflection coating which utilizes specified thicknesses of the first two layers of the coating to synthesize a layer having an effective index of refraction which is intermediate that of the first two layers. Again, Rock uses metal halides, oxides, sulfides, and selenides which are deposited in separate processing steps using vacuum deposition techniques.
Likewise, Rancourt et al, U.S. Pat. No. 4,229,066, teaches a visible light transmitting, infrared reflecting multilayer coating utilizing metal halides sulfides, and selenides. The materials have either a high or low index of refraction and are deposited in separate steps using vacuum deposition techniques. Neither of the coatings of Rock or Rancourt et al can be further shaped or formed after deposition.
Accordingly, the need still exists in this art for an optical interference film which can suppress a plurality of or multiple successive higher order reflections and yet be fabricated and then further shaped or post-formed using relatively inexpensive materials.